I've always been somewhat confused on the specifics of the sandworm life-cycle....some sources are contradictory (like the "encyclopedia"). Quinn mentioned some if it in a video back in 2017: https://www.youtube.com/watch?v=T52SL4TFWpU
I even rewatched and checked how the Dune miniseries explained it. I really hope you can do a follow-up at some point, straightening out how their life-cycle works. Rather, what confuses me is the "Sand plankton". To establish what we know with certainty: What's clear is that the "Little Makers / Sand Trout", larval Sandworms, are attracted to water, soak up and encyst water. They're slug-like, jellyfish-like creatures. They'd die if put on a true desert planet with no water at all. There is water on Dune, it's just all concentrated in and with the Little Maker nests, deep beneath the sands.
A "pre-spice mass" is caused by a Little Maker nest: eventually all the water they concentrate there mixes with their excretions/waste, then essentially ferments, until it explodes upward in a release of energy - spice blow. Many Little Makers die, but a few survive and metamorphize into Sandworms....and water is fatal to this stage. It takes hundreds if not thousands of years for new Sandworms to grow into the full adult "Shai-hulud" which rule the open deserts, and many do not survive fighting with their older cousins.
This is where the questions start: It's unclear how exactly Sandworms reproduce, sexually or asexually, eggs, whathaveyou, but this isn't terribly important. Little Makers..."ultimately' come from Sandworms.
What is sand plankton? Why do worms make spice? What do they eat? From the ramblings of Pardot Kynes in the Appendix:
Now they had the circular relationship: little maker to pre-spice mass; little maker to shai-hulud; shai-hulud to scatter the spice upon which fed microscopic creatures called sand plankton; the sand plankton, food for shai-hulud, growing, burrowing, becoming little makers. The second discovery concerned the sand-worm. These mighty creatures began their lives as sand plankton, then matured into the sandtrout form before becoming worm, It was the sandtrout phase — in which the "water-stealers" swam freely through the sand and sealed off all available water in the porous lower strata.
The Appendix makes it reasonably clear that "Sand Plankton" are in fact the FIRST stage of the sandworm life cycle. Protozoa that eventually mature into the jelly-fish like Sand Trout.
For a time, I wondered if the sand plankton ARE the spice, but the Appendix seems to state that they FEED on the spice. The adult Sandworms don't eat "Sand" as such, nor do they eat the spice....they eat the Sand Plankton growing on spice, spread throughout the sand.
The Spice is a natural byproduct of Little Makers plus Water. I'm not sure how new Sand Plankton get added to the mix. Are they just swirling around in the pre-spice mass, along with their older brothers who turned into Little Makers?
In which case, the life of a specific Sandworm looks something like this: adult Sandworm lays its nest deep beneath the sands, its spawn start out at Sand Plankton. The OLDEST of these that mature fastest advance to the Little Maker phase. Their excretions combine with water to make a pre-spice mass, which kills most of the Little Makers, though a few survive to become adult Sandworms. Meanwhile, the pre-spice mass explosion....spreads their younger siblings on the sand along with spice, which the plankton continues to feed on, until eventually eaten by adult Sandworms. New energy is added to this system....apparently?....from photosynthesis? Being absorbed by the sand plankton or just the chemical alteration of the new spice on contact with sunlight. I hope you see my confusion here. The chart that Matt Howarth drew in the Dune Encyclopedia (seen on their wiki page) doesn't make much sense.
And why are sandworms so fiercely territorial? Defending their own spice fields which they "farm"? Defending them against other sandworms? One practical benefit of the cycle is that Sandworms exhale oxygen as part of their internal chemical processes, explaining why Dune retains an Oxygen-rich, breathable atmosphere despite having such little plant life. It sort of reminds me of this 1995 scifi novel "The Legacy of Heorot", which deals with ecology issues on an alien planet: the colonists keep getting attacked by vaguely humanoid semi-aquatic reptile predators they dub "Grendels". They belatedly realize that the Grendel life-cycle involves preying on their own juvenile phase - a fish-like creature dubbed a "Samlon". When the colonists stocked all the local rivers with Terran fish species, it gave the adult Grendels something else to eat, so more Samlon survived to turn into Grendels, leading to a brief population explosion. The colonists then make a concerted effort to erradicate all the adult Grendels....only making matters worse, because this removed the keystone predator of this system: now EVEN MORE samlon survive to adulthood, in a massive population explosion. The ecology advisor on that book series was the same one as the War Against the Chtorr, another one dealing with a hypothetical alien ecology, where they only gradually realize that the "predator" species in this ecosystem are the adult stage of the "prey" species (only the fittest prey survive to be old enough to metamorphosize into the predator stage).